1. Field of the Invention
The present invention relates to a control circuit of a reactive power compensation apparatus for compensating a reactive power of variable load by a combination of a power capacitor and a thyristor control reactor connected in parallel to the variable load in a power system.
2. Description of the Prior Art
When variation of a reactive power is irregular as an arc furnace and variation of the period is fast, it is necessary to increase speed of a response of the compensation apparatus to variation of the reactive power in order to compensate effectively the variation of the reactive power. It is indispensable that the reactive power compensation apparatus comprises a reactive power detection circuit for detecting sudden variation of a reactive power of a load without delay and a gate circuit for determining immediately a turn-on phase angle of a thyristor depending upon the detected reactive power.
FIG. 1 shows a diagram of a main circuit of the conventional reactive power compensation apparatus.
In FIG. 1, the reference numeral (1) designates a leading capacitor having a leading phase reactive power Q.sub.c ; (2) designates a reactor and phase of the current fed to the reactor (2) is controlled by thyristors (3), (4) connected in series. The lagging reactive power Q.sub.R obtained from the power source by the reactor (2) under the phase control of the thyristors (3), (4) can be varied as desired in the range of 0 to 100%.
FIG. 2 shows currents fed to the reactor (2) and the thyristors (3), (4) by turning on the thyristors (3), (4) at a control angle .alpha. to power voltage V(.theta.). It is found that effective value of the current is varied depending upon the turn-on phase angle .alpha. of the thyristors (3), (4).
FIG. 3 shows relation of the turn-on phase angle .alpha. and the reactive power Q.sub.R fed to the reactor (2).
The lagging reactive power Q.sub.R fed to the reactor (2) can be controlled by varying the turn-on phase angles .alpha. of the thyristors (3), (4) whereby the leading reactive power can be controlled by combining with the capacitor (1).
For example, when it is considered that the leading reactive power of the capacitor is equal to the maximum lagging reactive power of the reactor (2) in the system of FIG. 1, the composite reactive power fed from the power source is given by the equation: EQU R.sub.F (.alpha.)=Q.sub.C -Q.sub.R (.alpha.) (1)
wherein R.sub.F (.alpha.) designates a reactive power fed to the reactive power compensation apparatus at the turn-on phase angle .alpha. of the thyristors (3), (4); Q.sub.c designates a leading reactive power fed to the capacitor (1) and Q.sub.R (.alpha.) designates a lagging reactive power fed to the reactor (2) at the turn-on phase angle .alpha..
As it is clear from the equation (1), the composite reactive power Q.sub.R (.alpha.) fed from the power source can be controlled in the range of 0 to 100% by varying the turn-on phase angle .alpha. of the thyristors (3), (4).
As shown in FIG. 3, the relation of Q.sub.F and .alpha. can be given by the equation; EQU Q.sub.F (.alpha.)=1-Q.sub.R (.alpha.)
wherein Q.sub.c =Q.sub.R (.alpha.=0)=1.0 per unit (P.U.). Thus, in the case of varying the lagging reactive power Q.sub.L for the variable load as shown in FIG. 1, the value Q.sub.L is detected and the leading composite reactive power Q.sub.F (.alpha.) being equal to the value Q.sub.L is fed from the power source to compensate the reactive power Q.sub.L for the load (5).
It has been well-known that when the reactive power for the load connected to a power system is varied, a terminal voltage for receiving the power is varied depending upon an impedance in the power system especially a reactance. The typical example of the load having large variation of a reactive power is an arc furnace. At the beginning of an operation of the arc furnace, a lagging reactive power having irregular period and fast variation and large width of variation is passed through the system whereby the voltage in the system connecting the arc furnace is seriously varied to cause voltage flicker and various disadvantageous effects such as flicker of light and racing of image of TV receiver are caused.
As a mechanism for controlling effectively such variation of the reactive power, the reactive power conpensation apparatus of FIG. 1 can be used. The reactive power Q.sub.L of the load (5) is detected and the thyristors (3), (4) are turned on at each 1/2 cycle of the power frequency at the turn-on phase angle .alpha. for generating the composite reactive power Q.sub.F which compensates the value Q.sub.L whereby the variation of the load reactive power is compensated or controlled. In such case, it is important to detect the variation of the load reactive power as soon as possible. The compensation effeciency of the reactive power compensation apparatus is determined by the speed for the detection of the variation.
FIG. 4 shows a control circuit of the conventional reactive power compensation apparatus wherein the reference numeral (6) designates a potential transformer for load voltage detection; (7) designates a current transformer for a load current detection; (8) designates a load resistor of the transformer (7) which is provided to obtain a voltage signal proportional to the current; (9) designates a control circuit which detects a load voltage, a load current or a load reactive power corresponding to the load voltage or the load current resulted by the transformer (6), the current transformer (7) and the load resistor (8) to determine the phase angle for turning on the thyristors (3), (4); (10) designates a gate circuit for feeding turn-on pulse to the gates of the thyristors (3), (4) at the turn-on phase angle determined by the control circuit (9).
The parts (1) to (5) are the same with those of FIG. 1 and the description thereof is not repeated.
The operation of the control circuit will be illustrated referring to the signal waveforms in FIG. 5.
The transformer (6) detects the load voltage v (.theta.) as shown in FIG. 5(a) and the current transformer (7) and the load resistor (8) detect the load current i.sub.L (.theta.) as shown in FIG. 5(a). The detected load voltage v(.theta.) and the load current i.sub.L (.theta.) are fed to the control circuit (9) which detects the instantaneous load current i.sub.L (n.pi./2) at the load voltage v(.theta.)=0 .theta.=n.pi./2, n=1, 2 . . . . The detected value is shown as Q.sub.A in FIG. 5(a) wherein q.sub.A is given by the equation; EQU q.sub.A =I.sub.p .multidot. sin .phi. (2)
wherein the reference .phi. designates a phase angle of the current i.sub.L at the voltage v and I.sub.p designates the maximum current in 1/2 cycle.
The load reactive power Q.sub.L is given by the equation; EQU Q.sub.L =VI.sub.p sin .phi.
The variation of the voltage is quite small. When the voltage is considered to be constant, the load reactive power Q.sub.L is proportional to I.sub.p sin .phi. and accordingly, the detected value q.sub.A of the control circuit (9) is proportional to the load reactive power Q.sub.L. Thus, the control circuit (9) detects the value q.sub.A at .theta.=.pi./2 as shown in FIG. 5(a) and the control angle .alpha. for generating the composite reactive power for compensation corresponding to the value q.sub.A is determined. The gate circuit (10) feeds the turn-on pulse to the thyristor (3) at 0=.pi.+.alpha..
The turn-on pulse is also fed to the thyristor (4) in the same principle. The description is not repeated.
There is the other method of detecting the reactive power value q.sub.B in the range of 0.ltoreq..theta..ltoreq..pi. or .pi..ltoreq.0.ltoreq.2.pi. as shown in FIG. 5(b) as the other detection of the control circuit (9).
The control circuit (9) forms a voltage v'(.theta.) lagging for 90 degree to the output signal v(.theta.) of the transformer (6) as shown in FIG. 5(b), and the following operations are performed in each of said sections; EQU q=.intg.v'(.theta.).times.i.sub.L (.theta.) d.sub..theta. (3)
The equation (3) is calculated depending upon the definition of the reactive power. The value q(n.pi.)=q.sub.B at .theta.=n.pi., n=1,2,3. . . is proportional to the reactive power in 1/2 cycle. Thus, the control circuit (9) detects the value q.sub.B at .theta.=n.pi. to determine the phase angle .alpha. for turning on the thyristor for generating compensation capacity corresponding to the detected value q.sub.B.
As shown in FIG. 5(a), the gate circuit (10) applies the turn-on pulse to the thyristor (3) at .theta.=.pi.+.alpha. whereby the reactive power Q.sub.L of the load (5) is compensated or controlled.
The dotted chain line in FIG. 5(a) shows the current i.sub.R (.theta.) passing through the thyristor (3).
In this method, the thyristor (4) is also turned on at suitable phase angle .alpha. in the same principle. The description is not repeated.
As it is clear from the above-mentioned description, in the conventional control circuit, there is a waste time from the detection of the load reactive power Q.sub.L and the compensation by the turn-on of the thyristor. The reactive power value in the last 1/2 cycle is compensated in the present 1/2 cycle. The 1/2 cycle lagging control is performed.
When the load current i.sub.L (.theta.) which has been 0 at .theta..sub.1 is suddenly increased in a phase difference of 90 degree to the load voltage v(.theta.) as shown in FIG. 6(a), the current in the last 1/2 cycle could not be detected and it is compensated in the next 1/2 cycle whereby the current in the last 1/2 cycle is remained.
This fact will be illustrated referring to the signal waveforms in FIG. 6. The detected value q.sub.A in the detection of FIG. 5(a) is shown in FIG. 6(b). The detected value q.sub.B in the detection of FIG. 5(b) is shown in FIG. 6(c).
In both cases, the reactive power in the term from .theta..sub.1 to .theta..sub.2 is detected to control the reactor current i.sub.R after .theta..sub.2. The reactor current i.sub.R is given as shown in FIG. 6(d). That is, the reactor current is equal to the capacitor current i.sub.c before .theta..sub.2 and the composite compensation current i.sub.F is zero as shown in FIG. 6(e). After .theta..sub.2, the values q.sub.A or Q.sub.B is detected to control the reactor current at the control angle .alpha. as the same with that shown in FIG. 6(d), whereby the compensation current i.sub.F starts after .theta..sub.2 to compensate the load current i.sub.L (.theta.) as shown in FIG. 6(e).
As it is clear from the above-mentioned fact, in the conventional control system, the compensation is started after .theta..sub.2 even though the load current is passed from .theta..sub.1. The compensation is started in lagging for 1/2 cycle. Therefore, the compensation for the first 1/2 cycle could not be performed to cause a compensation error.
When the load is seriously variated as an arc furnace, such current variation is repeated. In the conventional control circuit, the error caused by lagging the compensation can not be negligible and the compensation effect is inferior.
The thyristors (3), (4) are turned on at .theta.=.pi.+.alpha. by the reactive power detected at .theta.=.pi..
Even though sudden change of the current is caused after .theta.=.pi. as shown in FIG. 7, the detection of the reactive power has been finished whereby the sudden change of the current could not be detected. Therefore, the compensation apparatus compensates in the estimation that the load current has the waveform i.sub.L.sbsb.1 shown in FIG. 7. However, the waveform of the current which should be compensated is the waveform i.sub.L.sbsb.2 shown in FIG. 7 whereby a compensation error for the difference between i.sub.L.sbsb.1 and i.sub.L.sbsb.2 is caused to deteriorate the compensation effect.